You're going about it the wrong way

You need to use the HtmlElementCollection object with the GetElementsByTagName method, as in :

Code:
        private void button1_Click(object sender, EventArgs e)
        {
            System.Net.WebClient wc = new System.Net.WebClient();
            HtmlElementCollection imgs = this.webBrowser1.Document.GetElementsByTagName("img");

            for (int i = 0; i < imgs.Count; i++)
            {
                wc.DownloadFile(imgs[i].GetAttribute("src"), "c:\\images" + i.ToString() + ".jpg");
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            this.webBrowser1.Navigate("C:\\imagetest.htm");
        }
This downloads all the pictures to a folder on your C drive.